Passive radiator

It's a speaker with all the components removed including the
magnet. It is used to replace a port.

The correct way to design a passive radiator, is to run the
Thiele/Small parameters of the woofer and cabinet internal
volume, for a ported/bass reflex(ie:Fourth Order) alignment,
using the diameter of the radiator as, "port diameter." .
Then adjust the weight/mass of the passive radiator's cone, to
match the mass of the air that would have occupied the
resultant port's volume. A passive radiator should always be
1.5 to 2 times the diameter of the woofer.

And then the cabinet is basicly sealed?

The cabinet should be sealed, to avoid noise and messing up the alignment. It will, however, operate as a bass reflex system. "Sealed enclosure" indicates an Acoustic Suspension system.

Legacy Audio also makes use of Passive Radiators with very good results in some of its models....